In order to meet exhaust gas emission standards, the exhaust emitted from internal combustion engines is treated prior to emission into the atmosphere. Exhaust is passed through a catalytic element to remove undesirable gaseous emission components such as unburned hydrocarbons, carbon monoxide and nitrogen oxides. In addition to the gaseous components, exhaust gases also contain particulate matter such as carbon-containing particles or soot. A particulate filter, also commonly used with compression ignition engines, is used to prevent soot, or carbon particles, from exiting the tailpipe. Carbon particles are stored in the filter and then burned so that the filter is regenerated and able to again store the carbon particles. In a passive diesel particulate filter, the filter becomes so filled with particulate matter that the heat accumulated due to increased vehicle speed or load raises the temperature of the carbon to a temperature at which it ignites.
In the process to develop a catalyzed diesel particulate filter, it has been shown that eutectic salts such as KVO3 (potassium vanadate) and CsVO3 (cesium vanadate) are efficient diesel soot oxidation catalysts that provide lower soot ignition temperatures and higher soot combustion rates than other catalysts. These eutectic salts perform well as catalysts because of their low melting points, 520° C. for KVO3 and 641° C. for CsVO3 (see, for example, U.S. Pat. No. 4,515,758 to Domelse et al.). The low melting temperature of the eutectic salts causes them to be partially melted when the soot burns and this partial melting results in more efficient contact with the soot and thus efficient combustion. The low melting point of these eutectic salts, however, causes them to have high mobility when coated on a wall-flow diesel particulate filter. Under diesel engine exhaust treatment conditions, lack of durability as evidenced by loss of adhesion of the catalyst is a significant problem.
There thus remains a need for catalyst formulations for catalyzed diesel particulate filters that have good catalytic properties and low soot ignition temperatures.
The above described and other features are exemplified by the following figures and detailed description.